This post shows you how to fix "Assets file project.assets.json not found. Run a NuGet package restore" when rebuilding .NET Core in Visual Studio.
Assets file '<path>\project.assets.json' not found. Run a NuGet package restore to generate this file.
To fix this error you can select Tools menu => NuGet Package Manager => Package Manager Console, then enter the command as shown below.
restore nuget packages console
An error occurred because the dotnet cli didn't create all of the original required files. Therefore, you need to recover more necessary files.